Thoron (220Rn) may interfere with radon (222Rn) measurements, if present. We measured the thoron cross-interference (CI) signal of nine types of electronic radon instruments in constant thoron concentration without the presence of radon. The CI signal increases for the first 3 days of the exposure. Also, the initial interference signal may vary between instruments. Therefore, we propose a new test method for quantifying the thoron CI in radon measuring instruments. This includes exposure of the instrument in constant thoron concentration for a minimum of 3 days and fitting the acquired data in a simplified function, which will provide two parameters: initial and final CI.